Acceleration tube for microwave linear accelerator having an integral magnet structure



J. C. NYGARD TION TUBE FOR MICR LERATOR HAVING AN M NET F11 Oct Feb. 7, 1961 OWAVE LINEAR INTEGRAL ACCELERA ACCE STRUCTURE 17, 1957 ///////////,l||llI'//////////// /////AlIll-W///////// A .f 5 w //////////%||l|lIV/////// ///////%IIIIIIV////// ////AHHII IIHIIIW/ United States Patent ACCELERATION TUBE FOR MICROWAVE LINEAR ACCELERATOR HAVING AN INTEGRAL MAG- NET STRUCTURE John C. Nygard, Lexington, Mass., assignor to High Voltage Engineering Corporation, Burlington, Mass., a corporation of Massachusetts Filed Oct. 17, 1957, Ser. No. 690,818

1 Claim. (Cl. 315-3.5)

This invention relates to microwave linear accelerators for the acceleration of electrons to high energy and in particular to a novel construction of the acceleration tube of such a linear accelerator wherein the focusing of the electron beam is accomplished by a permanent magnet structure forming an integral part of the acceleration tube.

The invention may best be understood from the following detailed description thereof having reference to the accompanying drawing in which:

Fig. l is a diagram showing the major parts of a microwave linear accelerator;

Fig. 2 is a view in longitudinal central section of the accelerating tube of a microwave linear accelerator such as that shown in Fig. I;

Fig. 3 is a view similar to that of Fig. 2 but showing a modified orientation of the permanent magnets;

Fig. 4 is a perspective view of a portion of the accelcrating tube of a microwave linear accelerator such as that shown in Fig. 1, wherein alternating gradient focusing is accomplished by means of an integral magnet structure; and

Fig. 5 is a view similar to that of Fig. 4 but showing an alternative orientation of the magnetic fields involved.

The theory and construction of microwave linear accelerators is well known to those skilled in the art. Microwave linear accelerators are described, for example, by Walkinshaw at volume 61, pages 246254, by R. Shersby-I-Iarvie at volume 61, pages 255-270, and by Mullett and Loach at volume 61, pages 271-283 of The Proceedings of The Physical Society (1948). Referring to the drawings and first to Fig. 1 thereof, the conventional microwave linear accelerator comprises an accelerating tube 1 within which a travelling electromagnetic wave is produced by means of a radio-frequency power source 2. Electrons are injected into one end of the acceleration tube 1 from an electron injector 3. The traveling wave excited within the acceleration tube 1 is of such a nature that there is an axial electric field which travels the length of the tube 1 in phase with the moving electrons which are therefore accelerated to high energy and emerge at the opposite end 4 of the linear accelerator. In order to maintain the electrons as a focused beam, it has been customary to surround the waveguide 1 with external magnets of solenoidal type. In accordance with the invention, such magnets are eliminated and the wall of the acceleration tube 1 is itself adapted to provide the focusing magnetic field. In this way a more compact structure is achieved. Moreover, it is possible in accordance with the invention to make use of alternatnig gradient focusing.

Referring to Fig. 2, therein is shown the acceleration tube 1 of a conventional microwave linear accelerator which comprises essentially an iris-loaded waveguide. As in a conventional construction of iris-loaded waveguide, the waveguide 1 shown in Fig. 2, comprises a series of alternating spacer rings 5 and apertured disks or irises 6 which are held together in a vacuum-tight manner by being soldered together, or by mechanical pressure, or by any other well-known means. In accordance with the invention, however, the spacer rings 5 are made of permanent-magnet materials and may comprise, for example, an Alnico ring magnet which is copper plated, at least on its inner surface as shown at 7, or it may be copper-plated on all surfaces if desired. The irises 6 may be made of copper in the conventional manner. In the waveguide shown in Fig. 2, an axial magnetic field is created by the spacer rings 5 each of which has a north pole along its upper rim and a south pole along its lower rim, so that within the ring 5 the magnetic field is in the axial direction. As is well-known, the resulting axial magnetic field in the waveguide 1 gives the desired focusing effect.

Referring now to Fig. 3, therein is shown a waveguide structure 1 identical to that shown in Fig. 2 except for a rearrangement of the magnetic poles in the spacer rings 8. Each spacer ring 8 is identical in its entirety to the spacer rings 5 of Fig. 2, but the rings 8 are arranged in Fig. 3 so that the orientation of the magnetic field is reversed in alternate spacer rings 8. This still gives an axial magnetic field suitable for focusing electron beams.

Referring now to Fig. 4, the spacer rings 9 therein shown are similar to those shown in Figs. 2 and 3 except that the magnetic poles are rearranged in order to provide alternating-gradient focusing. There is no variation in the axial direction in the magnetic field within each spacer ring 8. The variation in the radial direction is such as to give the conventional quadrupole arrangement used in alternating-gradient focusing. In the arrangement of Fig. 4, successive magnets are rotated to give the alternating-gradient focusing action. Alternatinggradient lenses are well known in the art and are described, for example, in US. Patent No. 2,736,799 to Christofilos and at volume 88, pages 1190-1196, of the Physical Review.

Referring now to Fig. 5, the structure therein shown is similar to that of Fig. 4 except that each spacer ring 10 therein shown is the magnet equivalent of a pair of adjacent spacer rings of the type shown at 9 in Fig. 4. That is to say, the magnetic field within each spacer ring 10 is no longer axially uniform, but each north pole at one rim is opposed by a south pole at the opposite rim.

The appropriate magnetization of the various spacer rings may be carried out by well-known methods, and, in fact, many if not all of the herein described arrangements of magnetic poles correspond to arrangements in ring magnets currently manufactured by manufacturers of permanent magnets.

Having thus described the principles of the invention, together with several illustrative embodiments thereof, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense, and not for purposes of limitation, the scope of the invention being set forth in the following claim.

I claim:

Microwave linear accelerator comprising, in combination, an iris-loaded waveguide, means for producing a travelling electromagnetic wave in said waveguide, and means for injecting electrons into said waveguide in proper phase relationship with said travelling electromagnetic wave for acceleration of said electrons as an electron beam, said iris-loaded waveguide comprising alternating copper spacers and cylindrical magnets each longitudinally polarized so as to form two quadrupoles oppositely oriented, each such quadrupole comprising four magnetic poles symmetrically arranged about the axis of said waveguide, successive cylindrical magnets being arranged so that opposite poles are adjacent one another, the interior surface of each cylindrical magnet being copper plated, whereby space charge and other de- References Cited in the file of this patent UNITED STATES PATENTS Lindenblad Oct. 27, 1942 Woodyard Sept. 22, 1953 Rogers et a1. June 25, 1957 Miller et al. Oct. 22, 1957 Chodorow Nov. 19, 1957 Yasuda July 15, 1958 Cioffi -1 July 22, 1958 Pierce Aug. 12, 1958 Cook et a1 Nov. 11, 1958 Pierce Jan. 6, 1959 FOREIGN PATENTS France May 26, 1954 

